Catalysts which can be used for treating automotive exhaust are well known in the art. More specifically, these catalysts contain platinum and/or palladium and usually rhodium and are known as three-way catalysts (TWC). Stabilizers and promoters have also been used to improve the activity of these catalysts. For example, U.S. Pat. No. 4,528,279 describes the use of cerium/lanthanum promoters in conjunction with noble metals such as platinum and rhodium as the active phase of the catalyst. The purpose of adding promoters such as cerium or lanthanum is to improve the durability of the three component control catalysts after prolonged use on a vehicle. U.S. Pat. No. 4,591,580 teaches the use of a catalytic composite containing lanthanum oxide, cerium oxide and an alkali metal oxide.
It is well established that Pt/Rh catalysts are better at treating automotive exhaust than Pd/Rh catalysts. For example, Adams and Gandhi have reported that platinum oxidizes saturated hydrocarbons better than palladium and thus a platinum-containing catalyst is the only practical catalyst for automotive applications. K. M. Adams and H. S. Gandhi, Ind. Eng. Chem. Prod. Res. Dev., 22, 207-212 (1983). Further, the prior art shows that virtually all commercial automotive catalysts contain Pt and Rh. See C. H. Wu and R. H. Hammerle, Ind. Eng. Chem. Prod. Res. Dev., 22, 559-565 (1983); and H. Muraki et al., "Palladium-Lanthanum Catalysts for Automotive Emission Control". Ind. Eng. Chem. Prod. Res. Dev., 25, 202-208 (1986). Because of the price and availability of Pt versus Pd, it would be very desirable to use Pd instead of Pt in TWC. Additionally, Pd is domestically available whereas Pt is mostly available from politically volatile countries. Therefore, there is a pressing need for a Pd/Rh catalyst with equivalent performance to a Pt/Rh catalyst.
Applicant has discovered a catalyst formulation containing palladium and rhodium as the catalytic elements which has at least comparable activity to a platinum and rhodium containing catalyst. The Pd/Rh-containing catalyst has comparable or better activity not only in the fresh state, but after extensive thermal durability testing. The most pronounced improvement is in hydrocarbon and nitric oxide activity. These results have not previously been observed. The substantial increase in activity of the catalyst of this invention can result in substantial economic savings to automobile manufacturers since the platinum which costs about $600/troy ounce can be replaced by palladium which costs about $150/troy ounce.
In addition to Pt and Rh, the catalyst of the present invention also contains lanthanum oxide and cerium oxide all dispersed on an alumina support which in turn is deposited on a honeycomb monolithic carrier. The instant catalyst is characterized in that: (1) the cerium oxide has incorporated in its crystal lattice at least 3% of the lanthanum; (2) the lanthanum oxide is present as crystallites which are smaller than 25 Angstroms; and (3) the palladium is present as palladium oxide crystals which are smaller than about 25 Angstroms. It is believed that it is the combination of these three factors which has resulted in the observed substantial increase in activity.
In comparing the instant catalyst to catalysts of the prior art, it is noted that there is no mention of a Pd/Rh catalyst characterized in the manner of the instant invention. Also, there is no mention that a Pd/Rh catalyst may be made equivalent to a Pt/Rh catalyst by the addition of lanthanum in the controlled way disclosed by applicant. Therefore, applicant has discovered a novel catalyst which fills a long-felt need in the art.